(1) Field of the Invention
The present invention relates to an apparatus and process for the production of aluminum by electrolysis, and more particularly to an apparatus for aluminum smelting having separate sections for alumina dissolution and for electrolysis.
(2) Description of the Related Art
Aluminum is produced from bauxite, a mineral that contains various oxides and hydroxides of aluminum. In the most prominent commercial process, the aluminum values are extracted from bauxite ore by the Bayer process to produce alumina (Al2O3). Alumina is then processed to aluminum metal in the Hall-Héroult electrolytic process. About four tons of bauxite yield about two tons of alumina, which, in turn, yields about one ton of aluminum metal.
The Hall-Héroult process is an electrolytic process that uses electrical energy to split the aluminum and oxygen in aluminum oxide (alumina). In a typical Hall-Héroult electrolytic cell, an anode (positive electrode) made of carbon descends from the top of the cell, and a cathode (negative electrode) also currently made of carbon, forms the bottom of the cell. Both electrodes are submerged in a bath of molten cryolite electrolyte (sodium aluminum fluoride with added fluorides of calcium, aluminum, lithium and magnesium) at a temperature of about 960° C. Alumina has a limited solubility in molten cryolite, and most cells operate with 1.5-6% by weight aluminum oxide in the electrolyte. The carbon anodes are consumed during electrolysis and must be lowered during service to maintain a constant electrolyte gap of about 2-3 inches between the anode and the cathode. When the anodes are eroded to a certain degree, often after a period of only two or three weeks, the cell must be shut down for replacement of the anodes.
In the electrolytic process, aluminum metal is freed at the cathode and oxygen collects at the anode and reacts with the carbon of the anode to form carbon dioxide, which is vented from the cell. Aluminum metal forms a pool on top of the cathode and periodically is drained from the cell. As the concentration of alumina in the molten cryolite is depleted, it can be replenished by adding fresh alumina into the top of the cell.
The electrolysis process consumes large amounts of electricity, and about 15,000 to 16,000 kWh of electrical energy are required per ton of aluminum. Most of this energy is consumed in the electrolysis process, with much lower amounts used for the production of alumina. However, it is believed that most (up to 60%) of the electrical energy used during electrolysis goes toward the heating and melting of cryolite and the dissolution of alumina, rather than to the electrolytic splitting of aluminum and oxygen. Despite a great deal of research and development over the past 100 years, the Hall-Héroult production process, and the equipment used in that process commercially, has remained basically the same, and energy use efficiencies have not been improved by a great deal. See, e.g., Anonymous, Aluminum Industry, http://www.climatechangeindia.com/climatechange/aluminum.htm, Jan. 10, 2002; and Anonymous, New materials improve energy efficiency and reduce electricity use in aluminum production, http://es.epa.gov/techinfo/facts/nu-matrl.html, September 1992.
Efforts to improve the efficiency and operating characteristics of conventional Hall-Héroult electrolysis cells have included insulation of the cell, facing the cathode with a material that permitted cell operation with a lower inventory of molten aluminum (U.S. Pat. No. 4,650,552), providing a ceramic oxide coating for the anode (U.S. Pat. No. 4,173,518), designing cells with certain electrode configurations (U.S. Pat. Nos. 5,286,353, 5,006,209 and 4,865,701), compounding electrolytes that permit cell operation at different temperatures than normally used (U.S. Pat. Nos. 3,951,763 and 4,592,812), providing a highly agitated alumina feed area that is remote from the electrodes (U.S. Pat. No. 5,938,914), using cells having anodes with greatly increased surface area to permit lower temperature operation (U.S. Pat. No. 5,725,744), cooling the sidewalls of the cell to form a solid, protective layer (U.S. Pat. No. 4,608,135), or providing the cell with an inert liner (U.S. Pat. Nos. 4,608,134 and 4,608,135), and providing a cell in which the anode compartment and the cathode compartment are separated by a porous membrane (U.S. Pat. No. 4,338,177)
Efforts have also been made to improve the energy efficiency of the process by designing cells for enhanced heat recovery capabilities (U.S. Pat. No. 4,749,463), and by pre-heating alumina with heat recovered from the electrolysis cells (U.S. Pat. No. 4,451,337).
Other development efforts have focused on the provision of alternatives to the Hall-Héroult process. For example, U.S. Pat. No. 2,974,032 describes the reaction of alumina with carbon in an electric arc to produce aluminum and aluminum carbide, and different carbothermic processes are described in U.S. Pat. Nos. 3,971,653, 4,299,619 and 4,099,959. Other alternative processes are described in U.S. Pat. No. 5,505,823 (smelting aluminum from a potassium/aluminum sulfate mixture), in U.S. Pat. No. 4,445,934 (manufacturing aluminum by using a blast furnace), in U.S. Pat. No. 5,159,928 (smelting from a bath of aluminum chloride and using a tungsten plate or silicon carbide plate as the anode), in U.S. Pat. No. 4,324,585 (production of aluminum bromide and its subsequent electrolysis), and in U.S. Pat. No. 5,332,421 (smelting aluminum ore, such as nepheline syenite, with borax, sodium bicarbonate and a copper compound)
The Hall-Héroult electrolytic cell presently in commercial use carries out two distinct functions, the first is the mixing and melting of the components of the electrolytic bath, largely composed of cryolite, and the dissolution of alumina into the molten electrolyte. The second function of the cell is the electrolytic splitting of alumina into aluminum and oxygen. It is believed that present cell design is based on a compromise between the parameters that are important for each of these two different operations, and the cell is not optimized for either.
Separation of the operation of intermixing alumina with molten electrolyte from the electrolysis operation has been proposed in U.S. Pat. Nos. 3,501,387 and 3,616,439 to Love, which describes a charging cell that receives and intermixes alumina with molten electrolyte. The electrolyte with dissolved alumina is then circulated to a series of electrolysis cells where electrolysis is carried out. Molten electrolyte, depleted of alumina, is then recirculated back to the charging cell. In U.S. Pat. No. 4,681,671 to Duruz, an apparatus is described which recirculates molten electrolyte between an enrichment zone—in which fresh alumina is added—and an electrolytic cell having relatively large anode area, which is operated at a lower temperature than a normal Hall-Héroult process cell.
Modern Hall-Héroult cells are large and expensive to construct, resulting in financial charges amounting to more than the cost of alumina and power combined. The cells are operated in a series, or “potline”, of up to 130 cells. Such large operations are designed to operate under steady conditions and their efficiency suffers when a cell must be shut down for service, or if anode positions are not carefully monitored and controlled. Such installations are also not amenable to easy or efficient turn-down (operation at less than full capacity), and are very difficult to move from one location to another.
Despite the resources that have been devoted to the improvement of the aluminum production process, significant opportunity remains to decrease the operating and capital cost requirements of the equipment that is used for aluminum smelting. Furthermore, it would be useful to provide an apparatus and process for aluminum smelting that had a higher energy efficiency than the present process. It would be even more useful if such a process required a lower capital cost per unit of capacity. It would also be useful if such a process provided flexible scale-up and turn-down capabilities, and improved portability.